As strapdown inertial navigation technology continues to progress, there is a need to develop angular rate sensors which have the following characteristics: low cost, small size, lifetime calibration, no thermal control, high reliability, high bandwidth, and rapid reaction.
Prior art vibratory gyroscopes use tuning forks, vibrating beams or vibrating wires. However, their discrete nature has not proven easily adaptable to miniaturization or mass production.
Prior art U.S. Pat. No. 4,598,585 by Boxenhorn describes a planar single-input-axis vibratory gyroscope. Narrow sections of thin film define torsional flexures in a gimbal which supports an inertia member. By angularly oscillating the gimbal, an angular input rate normal to the plane causes the inertia member to vibrate. These oscillations are measured and output as a measure of the angular input rate. An elongated bar is attached to one side of the inertia member in order to make the instrument sensitive to an inertial input rate. The natural frequencies of the drive and sense axes are made to be substantially identical by varying the height to width ratio of the torsional flexures, making tuning dependent upon variations in flexure thickness and width. Because of variations in material thickness and undercutting, it is thus necessary to use discrete balancing weights in order to tune the manufactured sensor.
It is quite difficult to accurately control the material thickness in the micromachining of small structures made from silicon, quartz and other materials. The material thickness tends to vary somewhat from one batch to another. In the type of micromachining used by Boxenhorn, boron is diffused approximately 1 .mu.m into the silicon base material to define the outline and thickness of the torsional flexures. The heavily boron impregnated layer etches much slower than the base material during EDP etching and is therefore used as an etch stop. The boron diffusion process is difficult to control in order to obtain a uniform flexure thickness from one batch to another due to the many process parameters involved and the high temperature required. Another problem with the boron diffusion is that the width of the narrow torsional flexures is difficult to control due to sideways mobility of boron during the high temperature diffusion process. This tends to make the width of the flexures wider than the openings in the mask used to define the diffusion pattern.
Undercutting or underetching is a phenomena that occurs during etching in which the etching does not exactly follow the masking pattern. Underetching causes the etchant to delineate torsional flexures that have a different width than intended. The amount of underetching tends to vary somewhat from one batch to another.
The above problems cause the resulting flexures to vary in thickness, width and length from one batch to another. The different moments of inertia between the x- and y-axes in Boxenhorn's device due the presence of the gimbal and the elongated bar located on the inner element make it necessary to use torsional flexures of different widths for support of the gimbal and the inner element in order to obtain equal natural resonant frequencies. The ratio of the torsional stiffness of the flexures used in the x- and y-axis must have a predetermined ratio which depends on the ratio of moment of inertia about the same axes.
The required different flexure dimensions in Boxenhorn's design becomes a problem because of the processing problems cited. Variations in underetching and in sheet material thickness cause the nominally equal natural resonant frequencies about the x- and y-axes to differ from the desired value and from each other. Such a condition is unacceptable and requires costly and difficult trimming of the device in order to obtain equal natural resonance frequencies of the gimbal and the inertia member.
It is an object of this invention to reveal a new vibratory angular rate gyro. A further object is to provide a vibratory angular rate gyro where tuning using discrete balancing weights is not required since the drive and sense axes natural resonant frequencies are identical due to symmetry.